1. Field of the Invention
The invention relates to a delivery device with protection against freezing for delivering reducing agent into an exhaust-gas treatment device. The invention also relates to methods for operating and deactivating a delivery device and a motor vehicle having a delivery device.
The automotive field, in particular, uses exhaust-gas treatment devices to which a fluid is supplied for the purification of the exhaust gases. An exhaust-gas purification method particularly widely used in such exhaust-gas treatment devices is the process of selective catalytic reduction [SCR process]. In that process, reducing agent is added to the exhaust gas in order to reduce nitrogen oxide compounds in the exhaust gas. Ammonia is generally used as the reducing agent. Ammonia is normally not stored in motor vehicles directly but rather in the form of a precursor solution which can be converted to form ammonia in the exhaust-gas treatment device or in an additional reactor provided for that purpose. A particularly widely used reducing agent precursor solution is aqueous urea solution. A 32.5% urea-water solution available under the trademark AdBlue® is particularly widely used. For simplicity, the expressions “reducing agent” and “reducing agent precursor solution” will hereinafter be used synonymously for one another.
In constructing delivery devices for the delivery of (aqueous) reducing agent, it must be taken into consideration that the reducing agent can freeze at low temperatures. The reducing agent AdBlue®, for example, freezes at temperatures of −11° C. Such temperatures may arise, for example, during long standstill phases of motor vehicles. An aqueous reducing agent expands when it freezes. A delivery device must therefore be constructed in such a way that it is not damaged by the expansion of freezing reducing agent or by the associated pressure increase.
It is desirable for the delivery duct of the delivery device through which reducing agent is delivered to be substantially rigid for high dosing accuracy of a delivery device. In particular, if a dosing pump is used for the combined delivery and dosing of the reducing agent, a rigid delivery duct is used in order to ensure as small as possible a change in the volume of the delivery duct. It is only in that way that it can be ensured that the amount of reducing agent delivered by the dosing pump into the delivery duct corresponds precisely to the amount exiting the delivery duct.
In that case, there is accordingly a technical conflict with regard to the requirements for freezing resistance and duct stiffness, and it has heretofore not been possible to fully satisfactorily resolve the conflict, in particular in conjunction with high operational accuracy in dosing pumps.